So I did the book’s stem example (chapter 3, Oracle Text CONTAINS Query Operators, page 3-36) to make me sure is problem of my solution. And I change my preferences attributes INDEX_STEMS, STEMMER and FUZZY_MATCH values to ENGLISH, change my base table data to values same as the example:

The stem operator selects words that stem from the same root word, not just words that start with the same letters. I believe that they must match as to whether they are nouns or verbs and masculine or feminine as well. So, I believe you got the expeted results for the spanish words that you used. I added some additional words to the test below.

When you change the attributes of your preferences, it is not sufficient to rebuild the index. You need to drop the index and recreate it. When the index is created, it fetches the current attribute values and stores those values, not the preference names. So, if you only rebuild the index, it uses the old stored values. It only fetches the new values if you drop and recreate the index. I have demonstrated that below.